Going after the third line makes sense. Grabs-Hayes-Miller are incredibly dynamic offensively, but they tend to bleed shots defensively. They are constantly looking for an advantage to try to pounce and counter back up ice, but that makes them pretty susceptible in the defensive zone. If they read wrong and go too aggressive, it creates grade A scoring chances for the opposition.
Going after the third pairing, however, is probably not the same level of advantage. You would be better off targeting our 2nd pairing, where Staal and Holden tend to sit flatfooted and use their sticks to try and make plays, frequently allowing a skilled skater to dance circles around them. Galchenyuk could have a field day against them. The bottom pairing, on the other hand, has our best D-man not named Ryan McDonagh in the form of Brady Skjei and a perfectly capable defensive defenseman in the form of Brendan Smith. And even though Skjei is a rookie, you're not going to be able to take advantage of his inexperience. He played top minutes in the playoffs last year against the Penguins, where he was one of the very few bright spots of that campaign, so this isn't new to him.
Speaking of Brady Skjei, the only defensemen in the NHL to finish the season with more even strength assist than him? Duncan Keith, Erik Karlsson, and Brent Burns.
You have to dig deeper than that. After this season Matt Martin is one of 11 forwards to play 82 games and have a real gud pro factor of over 10.2RGP, the second highest real gud pro factor all-time is 9.8RGP.